SV40 large T antigen (LT) is a powerful model to study DNA replication and malignant transformation. We have identified an interaction of LT with the mitotic regulator Bub1. Bub1 is critical for preserving genome integrity. It primarily acts at the spindle checkpoint, where it serves a critical function in monitoring tension and attachment at sister chromatid kinetochores. Bub1 is mutated in certain human cancers characterized by aneuploidy. LT binding to Bub1 leads to attenuation of the spindle checkpoint. We hypothesize that binding of Bub1 to LT accounts for some of LT's ability to destabilize the host genome, in particular the ability to induce tetraploidy. Genetic analysis shows that LT binding to Bub1 is required not only for oncogenic transformation but also for viral DNA replication. Our analysis further demonstrates that LT binding to Bub1 is linked to p53 stabilization, which is likely a consequence of a DNA damage response. Our observations are consistent with a model whereby LT acts as a molecular scaffold to direct Bub1 to phosphorylate targets such as p53. The interaction with Bub1 suggests novel roles for LT that may shed light on how it causes chromosome aberrations, stabilization of p53 and transformation. We propose the following aims to study the interaction in detail: 1. Determine mechanistically how LT transforms via Bub1 and the potential contribution of a p53 gain of function 2. Characterize how LT uses Bub1 to induce tetraploidy and DNA damage responses 3. Elucidate how LT promotes viral replication via Bub1 binding. PUBLIC HEALTH RELEVANCE: Studies of the large T antigen of the SV40 DNA tumor virus have provided significant insight on basic mechanisms required for tumor induction and accurate duplication of DNA molecules (replication). Critical cellular targets such as the p53 tumor suppressor were first identified using this model. We have characterized a novel binding partner for T antigen called Bub1. Interestingly, Bub1 closely monitors and safeguards cell division, a process that T antigen has long been suspected to disrupt and thereby cause chromosomal abnormalities. Bub1 mutations are found in certain human cancers also characterized by chromosomal abnormalities. Our proposal will investigate how mechanistically T antigen binding to Bub1 is required for malignant conversion and replication. In the process, we expect to gain a better understanding of normal Bub1 function, and how its compromise facilitates tumorigenesis.